The present invention relates to an extrusion press device for manufacturing eccentric pipe sectionsxe2x80x94in particular pipe sections with circular outer and inner contoursxe2x80x94from extrusion blocks, in particular extrusion billets. The extrusion device features a container with a chamber with longitudinal axis MR which accommodates the extrusion block, an extrusion stem which is introduced into the container chamber, and features a dummy block, a mandrel body forming the inner wall of the pipe-section, and a die with an opening with longitudinal axis MM forming the outer wall of the pipe section. The invention further relates a process for manufacturing seamless, eccentric pipe sections, and the use thereof.
Pipe sections produced by means of extrusion processes are characterised by way of an outer and inner wall or an outer and inner contour of round cross-section. The outer and inner contours usually exhibit the same shape as viewed in cross-section.
The production of concentric pipe sections with a wall thickness which is essentially uniform, by means of extrusion, is known. Also known are extrusion processes which permit seamless concentric pipe sections to be produced. The expression xe2x80x9cconcentricxe2x80x9d indicates that the geometric middle points of the outer and inner contours as viewed in cross-section coincide with each other, with the result that when the outer and inner contours are of the same shape, the wall thickness over the whole cross-section is constant.
The production of seamless, concentric pipe sections is based on the principle of so-called extrusion over a mandrel. A mandrel body with mandrel arm and mandrel tip is advanced from a stem body, in the form of a hollow stem, into the container chamber and penetrates completely the extrusion block introduced into the container chamber. The mandrel tip is advanced up to or into the die opening immediately following the container chamber. The mandrel body does not have any points anchoring it to the die with the result that the extrusion block material is able to flow over the whole of the outer contour of the mandrel and into the die opening without forming a seam. In this process, because of the high flow stresses, the mandrel body cannot always be held exactly in the central position, the resultant pipe section is often not exactly concentric, as is desired, but instead slightly eccentric.
xe2x80x9cEccentricxe2x80x9d means that the geometric middle points of the outer and inner contoursxe2x80x94as viewed in cross-sectionxe2x80x94do not coincide, but instead lie a distance apart from each other and, accordingly, the thickness of the section wall varies over the cross-section.
The eccentricity of seamless, extruded pipe sections that are intended to be concentric is however very small, amounting to 0-10% of the average cross-sectional wall thickness of the section.
The eccentricity corresponds according its definition to the direct distance d between the two geometric centres of the outer and inner contour of the pipe section in cross-section.
For certain applications on the other hand use is made of pipe sections which are purposefully eccentric in cross-section. The eccentricity of such pipe sections is however, generally much greater than the process-related eccentricity values achieved with concentric designed pipe sections.
It is known to produce eccentric pipe sections by extrusion methods employing multi-chamber dies. The mandrel body is incorporated as a mandrel part in a die plate. The material to be extruded is fed to a welding chamber via a plurality of inlets under arms of the mandrel and, forming weld seams, passes around a shape-forming mandrel and through the die opening. Pipe sections manufactured by this process contain so called extrusion weld seams. This process is, however, suitable only for easily extrudable alloys exhibiting low mechanical strength values.
If the outer and inner contours have the same geometric shape, in particular that of a circle, then the eccentricity can be expressed by the following equation:                     E        =                                            S              max                        -                          S              min                                2                                    (        1        )            
where Smax represents the maximum and Smin the minimum thickness of the wall of the pipe section. The average wall thickness Sav of the eccentric pipe section in question can be calculated as follows:                               S          av                =                                            S              max                        +                          S              min                                2                                    (        2        )            
The magnitude of Sav also corresponds to the wall thickness of a concentric pipe section with the same outer and inner contour measurements as the eccentric pipe section.
To compare the eccentricities of pipe sections of various sizes, i.e. such sections with different outer and inner contour measurements, the so called relative eccentricity ER is calculated as follows:
ER=E/Savxe2x80x83xe2x80x83(3)
Whereas the continuous manufacture of seamless extruded concentric pipe sections is practised on an industrial scale, the production of seamless, eccentric pipe sections with constant eccentricity along the lengthxe2x80x94allowing for a tolerance rangexe2x80x94has not yet been solved satisfactorily.
Trials aimed at the production of seamless, intentionally eccentric pipe sections by extrusion over a mandrel, result in the mandrel arm usually being deflected towards the middle of the die opening as a result of the different flow pressures over the cross-section. This results in pipe sections with eccentricity values that deviate significantly from the intended values and non-uniformly along the length of the section; these eccentricities lie far beyond the normal inaccuracy of 0 to 10% of the average wall thickness. The deflection of the mandrel arm towards the centre of the die opening can, furthermore, lead to parts of the extrusion press device being damaged. Also, eccentric pipe sections manufactured this way tend to bend and curve when they emerge from the die. This means that the final length of pipe section bends off to one side on leaving the die.
The object of the present invention is to propose an extrusion press device and an extrusion process for manufacturing seamless, eccentric pipe sections having as constant as possible eccentricity along their length.
That objective is achieved by way of the invention in that the mandrel body, when in the position for extrusion, is a mandrel arm of longitudinal axis MD with a mandrel tip that can be pushed out of the dummy block through the extrusion block up to or into the die opening, such that the material of the extrusion block can flow in a seamless manner around the mandrel arm, through the die opening. The mandrel arm is arranged eccentric in cross-section with respect to the container chamber and with respect to the die opening, and the die opening is arranged eccentric in cross-section with respect to the container chamber. The longitudinal axis MD of the mandrel arm and the longitudinal axis MR of the container are a distance apart and lie essentially parallel to the longitudinal axis MK of the die opening, in such a way that the longitudinal axis MK of the die opening in cross-section lies between a pair of straight lines g1 and g2 each passing through the mandrel arm longitudinal axis MD and the container chamber longitudinal axis MR of as well as perpendicular to lines p connecting the mandrel arm longitudinal axis MD and the container chamber longitudinal axis MR.
The container chamber longitudinal axis MR, the mandrel arm longitudinal axis MD and the die opening longitudinal axis MK are so called middle axes which in cross-section pass through the geometric middle point of the elements of the device.
The mandrel arm longitudinal axis MD, the container chamber longitudinal axis MR and the die opening longitudinal axis MK are preferably parallel to each other.
In a preferred version the eccentric arrangement of the mandrel arm with respect to the container chamber and the die opening, and the arrangement of the die opening with respect to the container chamber are chosen such that the container chamber longitudinal axis MR, the mandrel arm longitudinal axis MD and the die opening longitudinal axis MK lie on a common plane and parallel to each other, and the die opening longitudinal axis MK lies in cross-section between the container chamber longitudinal axis MR and the mandrel arm longitudinal axis MD. That is, the die opening longitudinal axis MK lies, as viewed in cross-section, on straight lines p connecting the chamber longitudinal axis MR and the mandrel arm longitudinal axis MD.
In a particularly preferred version the relative eccentricity ERr of the hollow cylinder shaped, bored extrusion block corresponds to the relative eccentricity ERm of the pipe section or extrusion.
The die axis MM itself also preferably lies concentric with the container chamber axis MR, i.e. the die opening lies eccentric with respect to the outer contour of the die.
The die i.e. the die opening is with respect to the container, i.e. the container chamber, preferably fixed and unmoveable.
The extrusion block is preferably a circular cylindrical-shaped billet. The container chamber is likewise circular cylindrical-shaped.
The device according to the invention is particularly suitable for manufacturing pipe sections of circular outer and inner contours, whereby the shaping wall of the mandrel arm and the shaping wall of the die opening are circular in cross-section.
The extrusion press device according to the invention is particularly suitable for extruding extrusion blocks made of metallic materials, especially such as aluminum or aluminum alloys, such as aluminum wrought alloys.
In the extrusion press device according to the invention, the mandrel arm which forms the inner contour of the pipe section during extrusion is not part of the extrusion die and therefore is not anchored to the die, but instead is provided as a hollow stem on the stem body and, prior to the actual extrusion process, is moved out of the dummy block of the stem body up to the extrusion block in the container chamber whereby the mandrel arm penetrates completely the extrusion block in the container chamber in the direction of extrusion.
The mandrel arm may be of the kind that moves in the extrusion direction during the extrusion process or it may be fixed in place. The extrusion process may also be indirect extrusion or, preferably, a direct extrusion process. Usefully, the mandrel arm also features a mandrel tip which engages with or enters into the die, said tip having a slightly smaller diameter than the rear part of the mandrel. The diameter dt of the mandrel tip is less than 10%, in particular less than 5%, smaller than the diameter DT of the rear part of the mandrel arm.
The mandrel tip of the mandrel arm is moved up to or into the die opening. In the direct extrusion process the stem body is then advanced and the extrusion block material pressed through the die. The extrusion block material is thereby pressed around the mandrel arm and flows in the direction of extrusion ring-like along the mandrel arm and through the die opening without forming a seam. The mandrel tip situated in the die region produces the final shape of the inner wall of the pipe section being produced, whereas the die opening produces the final shape of the outer wall of the pipe section. The extrusion block shaped in the die emerges from the die as a seamless, eccentric pipe section. By means of the eccentric arrangement of the mandrel, container chamber and die opening according to the invention, one obtains a uniform distribution of the extrusion or flow pressure around the mandrel arm which lies free in the container chamber, with the result that it is not displaced from its original position during extrusion. Furthermore, because of the extrusion press device according to the invention, the rates of flow of the extrusion block material i.e. extrudate within the die opening is uniform, with the result that the emerging pipe section does not bend to the side.
In the following, with the aid of a preferred embodiment of the inventive device, the technical operation of the invention is explained. The container chamber longitudinal axis MR, the mandrel arm longitudinal axis MD and the die opening longitudinal axis MK lie in a common plane and parallel to each other, whereby the die opening longitudinal axis MK lies in cross-section between the container chamber longitudinal axis MR and the mandrel arm longitudinal axis MD.
The descriptions refer to the production of pipe sections having circular inner and outer contours using a cylindrical shaped extrudate in a container chamber of the same shape.
As described above, the flow rates in the container chamber and in the die opening and the pressure or flow forces acting on the mandrel body must be constant over the relevant cross-section, in order to be able to extrude seamless, concentric or eccentric pipe sections.
These process parameters may, according to the invention, be controlled by varying the width of cross-sectional flow in the container chamber.
During extrusion, the stem and with that the extrudate in the container chamber moves in the direction of extrusion at a rate of v1. In the through-flow cross-section in the container chamber exhibiting the smallest radial distance A between the mandrel arm and the container wall, i.e. in the region with the smallest through-flow cross-section, the through-flow of extrudate amounts to A*v1. In the through-flow cross-section in the container chamber exhibiting the largest radial distance B between the mandrel arm and the container wall, i.e. in the region with the largest through-flow cross-section, the extrudate flow amounts to B*v1.
In order to prevent the extrusion from bending to the side when it leaves the die, the extrudate must move with a uniform flow rate v2 across its cross-section. The flow of extrudate material in the through-flow cross-section with the smallest radial distance a, which lies along the line of the through-flow cross-section A, amounts, therefore, between the mandrel arm and the die opening wall to a*v2. The through-flow in the through-flow cross-section with the largest radial distance b, which lies along the line of the through-flow cross-section B, amounts, therefore, between the mandrel arm and the die opening wall to b*v2.
As the extrudate material cannot be compressed, and there should be no flow of material around the mandrel arm in the container transverse to the direction of extrusion, the through-flow A*v2 of extrudate at the smallest width of through-flow cross-section in the container should correspond to the through-flow a*v2 of the extrudate at the smallest width of through-flow cross-section in the die opening, and the through-flow B*v1 of the extrudate at the largest width of through-flow cross-section in the container corresponds to the through-flow b*v2 of extrudate at the largest width of through-flow cross-section in the die opening.
As a result the following set of equations is obtained:
Axc3x97v1=axc3x97v2xe2x80x83xe2x80x83(4)
Bxc3x97v1=bxc3x97v2xe2x80x83xe2x80x83(5)
From this the following relationship can be derived:
A/B=a/bxe2x80x83xe2x80x83(6)
The ratio A/B of the smallest radial distance A to the largest radial distance B between the mandrel surface and the container chamber wall corresponds therefore to the ratio a/b of the smallest radial distance a to the largest radial distance b between the mandrel arm surface and the die opening wall.
The equation (6) expresses, amongst other things, the condition that the relative eccentricity ERr of the hollow cylindrical shaped, bored extrudate block should correspond to the relative eccentricity ERm of the pipe section or extrusion. The xe2x80x9cwall thicknessxe2x80x9d according to equations (1) and (2) for calculating the relative eccentricity ERm correspond here to the radial distances between the surface of the mandrel arm and the wall of the container chamber or the wall of the die opening.
On the basis of the above, the relative eccentricity ERr of the mandrel body with respect to the container chamber usefully deviates by less than 10%, advantageously less than 5%, in particular less than 2%, from the relative eccentricity ERm of the mandrel arm with respect to the die.
The more accurately the conditions formulated in equation (6) are observed, the less the mandrel arm is displaced towards the die opening longitudinal axis, and the smaller is the deviation of the effective eccentricity of the pipe section produced compared to its intended values. Further, by observing the above conditions, the eccentricity of the pipe section remains constant over the length of the pipe section.
Also in the case of pipe sections designed with eccentricity it is necessary to reckon with small fluctuations in eccentricity along the section length. These fluctuations in eccentricity amount toxe2x80x94as with seamless concentric pipesxe2x80x94at most 0 to 10% of the average wall thickness Sav of the pipe-section, and is sufficient to meet the requirements regarding tolerances for seamless, eccentric pipe sections.
The device according to the invention is suitable also for manufacturing pipe sections of e.g. an elliptical, oval or some other shape, in particular roundish, or polygonal cross-section. The device may also be employed for producing pipe sections with different geometrical outer and inner contours as viewed in cross-section. Observing the previously mentioned condition viz.,
A/B=a/b
as accurately as possible is also in such cases decisive for successful production i.e. for a good quality of final product.
Also within the scope of the invention is an extrusion process for manufacturing seamless, eccentric pipe sections from extrusion blocks, in particular extrusion billets, using the extrusion press device discussed above.
The extrusion process according to the invention is characterised in that the extrusion block is pushed to the die end face by means of an extrusion stem and the mandrel arm is driven from the dummy block into the extrusion block and pushed by the mandrel tip in a position eccentric with respect to the die opening up to or into the die opening, whereby the mandrel arm is pushed through the extrusion block in an eccentric position and the extrusion block is pushed through the die by the extrusion stem, in such a manner that the extrusion block material flows without forming a seam over the whole cross-section at uniform speed around the mandrel tip into the die opening.
The mandrel arm is preferably moved in an eccentric position with a relative eccentricity ERr to the container chamber and in an eccentric position with a relative eccentricity ERm to the die opening and the relative eccentricity ERr corresponds essentially, preferably exactly, to the relative eccentricity ERm. The longitudinal axis MK of the die opening, the mandrel longitudinal axis MD and the container chamber longitudinal axis MR in cross-section usefully lie on the same plane.
The process is suitable in particular for extruding metallic materials, in particular aluminium or aluminium alloys such as aluminium wrought alloys.
Seamless eccentric pipe sections manufactured using the process according to the invention can e.g. be employed as, or processed further into, support sections which are subjected to directional, in particular one dimensional, loads. The region with maximum wall thickness is situated in the zone where the largest extension forces are present due to bending. Eccentric pipes designed to bear bending forces are, for the same load bearing capacity, much lighter than concentric pipes. Furthermore, eccentric pipe sections are particularly suitable for manufacturing bent pipe-sections e.g. elbow-joint lengths. To that end, the eccentric pipe section is bent in such a manner that the thick-walled part of the pipe is situated in the zone undergoing elongation and the thin-walled part of the pipe is situated in the compression zone. In the elongated region therefore, there is excess material available, which is necessary for elongation purposes. As a result of the thicker wall section no critical thinning of the pipe wall occurs on the outer side of the pipe section. On the other hand, the wall can be thinner the compression zone as the wall is not stretched there. If concentric pipe sections are employed for this purpose, then the wall thickness must be chosen with regard to the part undergoing the largest forces i.e. the stretched part. This means that in other parts undergoing compression, the wall thickness is over-dimensioned. By using eccentric pipe sections instead of concentric pipe sections, weight can be saved while maintaining the same mechanical properties.
The eccentric shape of the pipe section guarantees a continuous transition from the wall thickening to the wall thinning. Similarly on bending the pipe there is a continuous transition from stretching to compression, whereby in the neutral zone i.e. where there is neither stretching nor compression, the thickness of the pipe section has the average thickness of the eccentric pipe section.
Seamless eccentric pipes are especially suitable for manufacturing U-shaped rear axle supports for private cars. Hydrostatic forming, i.e. shaping with high internal pressure, is particularly suitable for shaping such pipe sections.
The seamless eccentric pipe sections manufactured using the device according to the invention may be shape-formed or bent e.g. using hydrostatic forming or other cold forming processes. Pipe sections conceived with eccentric cross-sections are generally suited to forming processes employing high internal pressure, in which the wall regions are stretched to different degrees. With eccentric pipe sections material can be specifically made available for the stretching regions, while in regions which are subjected to less the wall section can be thinner.
Compared with eccentric pipe sections produced using multi-chamber dies, the seamless eccentric pipe sections do not have any weaknesses such as extrusion welds.
The above mentioned eccentric pipe sections may e.g. exhibit an outer diameter of 10 to 500 cm, in particular 10 to 100 cm, and wall thicknesses of 1 to 50 cm, in particular 1 to 10 cm.